Answer:
a) 16.2 dm^3/mol*h
b) 6.1 × 10^3 s, 2.5 × 10^3 s (it is different to the hint)
Explanation:
We can use the integrated rate equation in order to obtain k.
For the reaction A+ B --> P the reaction rate is written as
Rate = 
If
and
are the inital concentrations and x the concentration reacted at time t, so
and
and the rate at time t is written as:

Separating variables and integrating

The integral in left side is solved by partial fractions, it can be used integral tables

Using logarithm properties (ln x - ln y = ln(x/y))

Using the given values k can be calculated. But the data seems inconsistent since if the concentration of A changes from 0.075 to 0.02 mol dm^-3 it implies that 0.055 mol dm^-3 of A have reacted after 1 h, so according to the reaction given the same quantity of B should react, and we only have a
of 0.05 mol dm^-3.
Assuming that the concentration of B fall to 0.02 mol dm^-3 (and not the concentration the A). So we arrive to the answer given in the Hint.
So, the values given are t= 1,
,
,
, it implies that the quantity reacted, x, is 0.03 and
. Then, the value of k would be


b) the question b requires calculate the time when the concentration of the specie is half of the initial concentration.
For reactant A, It is solved with the same equation

but suppossing that
so
, k=16.2 and the same initial concentrations. Replacing in the equation

For reactant B,
so
, k=16.2 and the same initial concentrations. Replacing in the equation
Note: The procedure presented is correct, despite of the answer be something different to the given in the hint, I obtain that result if the k is 19.2... (maybe an error in calculation of given numbers)